Hubble Space Telescope spots clouds on alien worlds

Artist's view of extrasolar planet GJ1214b (Nasa, ESA, and D. Aguilar (CfA))

Astronomers have spotted clouds in the atmospheres of two planets outside our solar system, with a pair of research papers suggesting that these sorts of cloudy alien worlds might be far more common than previously thought.

Two separate research teams examined data from the Hubble Space Telescope relating to the extrasolar planets GJ 1214b and GJ 436b. Located 42 light-years away from Earth, GJ 1214b is a prime example of a ‘super-Earth’ planet whilst GJ 436b - 36 light-years away - is a ‘warm Neptune’.

Super-Earths are planets with a mass between that of Earth and Neptune (GJ 1214b has a radius 2.7 times that of Earth) whilst warm Neptunes are, as the name suggests, hot versions of our own Neptune. They maintain a close orbit to their star of less than one astronomical unit (roughly the average distance from the Earth to the Sun, or around 90,000,000 miles).

“We always knew the clouds must be there for some planets, but now we have a wave of results telling us that clouds are actually very common,” said Heather Knutson, a planetary astronomer at the California Institute of Technology in Pasadena and lead author of the paper studying GJ436b, to Space.com.

The composition of these planets are studied when they pass in front of their stars. When this ‘transit’ occurs, the molecules that make up the exoplanet’s atmosphere block off different parts of the light spectrum. By recording which sections make it through and which don’t astronomers can deduce what the planets are made of.

For GJ 1214b these observations were notable for revealing no signs of an atmosphere at all. It was thought that this could mean that thick clouds were blocking the starlight or that the planet was made up of relatively heavy molecules such as water, with gravity compressing this substance into a thin, dense layer.

This most recent study, led by Laura Kreidberg of the University of Chicago in Illinois, has shown that there “have to be clouds” present. However, these would be utterly alien to the clouds found on Earth, and are likely to be made up of compounds such as zinc sulphide or potassium chloride, rather than our own, comparatively homely, mixture of water droplets and ice.